When a keyboard is used to control a synthesizer, it provides three major control signals: NOTE, GATE and VELOCITY. The NOTE signal correspoonds to the key depressed and determines the pitch of the final sound. The GATE decides when the sound is initiated and stopped, which corresponds to the instant of key depression (for initiating the sound) and the instant key release (for stopping the sound). The VELOCITY is a parameter which is proportional to the force with which the key was struck. This may be interpreted by the synthesizer as the volume of the sound or can be used to control other timbral characteristics of the sound so that the dynamics are a direct function of the force of strike.
In a guitar synthesizer controller, preferably such as that which I have described in my copending patent application Ser. No. 669,666 filed Nov. 8, 1984 now U.S. Pat. No. 4,630,520, NOTE, GATE and VELOCITY signals must also be generated and must be derived from the normal guitar plating technique and made available for proper synthesizer operation.
In the case of a guitar controller NOTE information is determined by which string is depressed and the particular fret at which depression occurs. The method by which this information may be derived on the guitar controller previously disclosed is clearly outlined in that application.
The standard method of deriving GATE information on a guitar is to process the vibrating string through an envelope detector. However, I have discovered through experimentation, use of guitar controllers on the market and by reading published literature, that this method is inadequate for several reasons.
All of these earlier systems are fraught with various problems and drawbacks obviated by the present invention and some of which will be detailed below.
First, the speed with which an envelope detector responds to the onset of a plucked vibrating string depends on the fundamental frequency of that string, for example as described in Meno, U.S. Pat. No. 4,430,918. For instance, the Low E string on a guitar has a period of 12 milliseconds. This signal can theoretically be detected within one half cycle by an ideal envelope detector using full wave rectification. Thus, the fastest response possible for detection of the first vibratory peak would be 6 milliseconds (ms) on the low E string. This delay is detectable by the guitar player as a lag in response from the pick to the sound generated by the synthesizer. Admittedly, this minimum response time is less for higher pitched strings, however the problem becomes even more involved upon further examination. In addition, the lower frequencies of bass guitar strings makes this method totally unacceptable for bass guitar purposes.
The vibration characteristics of a guitar string are very complex. For instance, because of beating of nonharmonic overtones, the string vibration does not always reach its full peak of oscillation until well into its third or fourth cycle. Thus, any attempt at deriving a VELOCITY signal from a peak detector that follows and senses the peak of the envelope contour can cause a delay of 20 to 40 ms, which is totally unacceptable.
A further problem arises because GATE and VELOCITY information must be transmitted in immediate succession to conform with normal synthesizer protocols. Since the GATE is typically derived from the immediate rise of the envelope while the VELOCITY peak may be delayed by many milliseconds, it becomes obvious that either the GATE must be delayed to conform with the VELOCITY or the VELOCITY information must be forfeited. Thus, the standard method of deriving VELOCITY from an envelope is actually impossible.
Lastly, the complex shape of a typical guitar string vibration causes false peaks and valleys within one cycle. Thus, a fast envelope detector can actually be "fooled" into thinking that it has reached a peak of vibration when it has actually only captured a contour of the string vibration.